Axios interceptors in Vue - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an Axios interceptor?

An Axios interceptor is a function that runs before a request is sent or after a response is received. It lets you modify or handle requests and responses globally in one place.

beginner

How do you add a request interceptor in Axios?

Use axios.interceptors.request.use() and pass a function that receives the request config. You can change the config or add headers before the request is sent.

beginner

What is the purpose of a response interceptor in Axios?

A response interceptor lets you handle or modify the response data before your app uses it. You can also catch errors globally here.

intermediate

How do you remove an Axios interceptor?

When you add an interceptor, it returns an ID. Use axios.interceptors.request.eject(id) or axios.interceptors.response.eject(id) to remove it.

beginner

Why use Axios interceptors in a Vue app?

Interceptors help keep your code clean by handling things like adding auth tokens, logging, or error handling in one place instead of repeating it everywhere.

Which method adds a request interceptor in Axios?

A axios.interceptors.request.use()

B axios.addRequestInterceptor()

C axios.request.interceptor()

D axios.useRequestInterceptor()

The correct method to add a request interceptor is axios.interceptors.request.use().

What can you do inside an Axios response interceptor?

A Cancel the request

B Change the request URL

C Add headers to the request

D Modify response data before it reaches your code

Response interceptors let you modify or handle the response data before your app uses it.

How do you remove an Axios interceptor?

A axios.interceptors.request.eject(id)

B axios.interceptors.request.remove(id)

C axios.interceptors.eject(id)

D axios.removeInterceptor(id)

You remove a request interceptor with axios.interceptors.request.eject(id) and a response interceptor with axios.interceptors.response.eject(id).

Why are Axios interceptors useful in Vue apps?

A They speed up the app by caching requests

B They let you handle common tasks like auth or errors in one place

C They replace Vue lifecycle hooks

D They automatically update the UI

Interceptors help keep code clean by handling common tasks like authentication or error handling globally.

What arguments does axios.interceptors.request.use() accept?

A Only an error function

B Only a success function

C A function for success and a function for error

D No arguments

It accepts two functions: one for handling the request config on success, and one for handling errors.

Explain how to use Axios interceptors to add an authorization token to every request in a Vue app.

Think about changing the request config before it is sent.

Describe how you can globally handle HTTP errors using Axios interceptors in Vue.

Focus on what happens after a response is received.

Practice

1. What is the main purpose of Axios interceptors in a Vue application?

easy

A. To modify requests or responses globally before they reach your components

B. To create new Axios instances for each API call

C. To replace Vue lifecycle hooks with Axios methods

D. To automatically refresh the Vue component state

Solution

1. Step 1: Understand Axios interceptors role

   Axios interceptors allow you to intercept and modify requests or responses globally before they reach your Vue components.

2. Step 2: Compare options

   Options B, C, and D describe unrelated or incorrect uses of Axios interceptors.

3. Final Answer:

   To modify requests or responses globally before they reach your components -> Option A

4. Quick Check:

   Interceptor purpose = Modify requests/responses globally [OK]

Hint: Interceptors act like global filters for requests and responses [OK]

Common Mistakes:

• Thinking interceptors create new Axios instances
• Confusing interceptors with Vue lifecycle hooks
• Assuming interceptors update component state automatically

2. Which of the following is the correct syntax to add a request interceptor using Axios in Vue?

easy

A. axios.use.interceptors.request(config => { return config })

B. axios.request.interceptors.use(config => { return config })

C. axios.interceptors.request.use(config => { return config })

D. axios.interceptors.use.request(config => { return config })

Solution

1. Step 1: Recall Axios interceptor syntax

   The correct syntax to add a request interceptor is axios.interceptors.request.use(callback).

2. Step 2: Identify correct option

   Only axios.interceptors.request.use(config => { return config }) matches the correct syntax; others have incorrect property order or method calls.

3. Final Answer:

   axios.interceptors.request.use(config => { return config }) -> Option C

4. Quick Check:

   Correct syntax = axios.interceptors.request.use [OK]

Hint: Remember: interceptors.request.use is the correct chain [OK]

Common Mistakes:

• Swapping 'request' and 'interceptors' order
• Using 'use' before 'interceptors'
• Missing the return of config in the callback

3. Given this Axios response interceptor code in Vue:

axios.interceptors.response.use(response => {
  response.data.modified = true;
  return response;
});

const result = await axios.get('/api/data');
console.log(result.data.modified);

What will be logged to the console?

medium

A. undefined

B. true

C. false

D. An error will occur

Solution

1. Step 1: Analyze the response interceptor

   The interceptor adds a property 'modified' with value true to response.data before returning it.

2. Step 2: Understand the logged value

   After the GET request, result.data.modified will be true because the interceptor modified it.

3. Final Answer:

   true -> Option B

4. Quick Check:

   Interceptor modifies data = logged true [OK]

Hint: Interceptors can add properties to response data before use [OK]

Common Mistakes:

• Assuming interceptor does not modify response
• Expecting undefined because property was added dynamically
• Thinking an error occurs due to property addition

4. Identify the error in this Vue Axios interceptor code:

axios.interceptors.request.use(config => {
  config.headers['Authorization'] = 'Bearer token';
});

medium

A. Missing return of config in the interceptor callback

B. Headers should be set on response, not config

C. Authorization header value must be a function

D. Interceptor should be added on axios.request, not axios.interceptors

Solution

1. Step 1: Check interceptor callback requirements

   Axios interceptors must return the config object after modifying it to continue the request chain.

2. Step 2: Identify missing return

   The code modifies headers but does not return config, causing the request to hang or fail.

3. Final Answer:

   Missing return of config in the interceptor callback -> Option A

4. Quick Check:

   Always return config in request interceptor [OK]

Hint: Always return config or response in interceptors [OK]

Common Mistakes:

• Forgetting to return config or response
• Setting headers on response instead of request config
• Misplacing interceptor on wrong Axios property

5. You want to globally handle 401 Unauthorized errors in your Vue app using Axios interceptors. Which approach correctly implements this behavior?

hard

A. Use a response interceptor to retry the request infinitely on 401 errors

B. Use a request interceptor to modify the request URL on 401 errors

C. Use a request interceptor to add a 401 status header before sending

D. Use a response interceptor to check error response status and redirect to login

Solution

1. Step 1: Understand 401 error handling

   401 Unauthorized errors come from server responses, so handling must be in a response interceptor.

2. Step 2: Choose correct handling method

   Use a response interceptor to check error response status and redirect to login correctly uses a response interceptor to detect 401 status and redirect user to login page.

3. Step 3: Eliminate incorrect options

   Retrying infinitely causes loops; using a request interceptor cannot handle response errors like 401; adding a 401 status header in the request is invalid.

4. Final Answer:

   Use a response interceptor to check error response status and redirect to login -> Option D

5. Quick Check:

   401 errors handled in response interceptor [OK]

Hint: Handle server errors in response interceptors, not request ones [OK]

Common Mistakes:

• Trying to handle response errors in request interceptors
• Retrying requests infinitely on 401 errors
• Adding fake status headers in requests